diff --git a/source/ecs/manager.d b/source/ecs/manager.d
index 36acd5e..56e887c 100644
--- a/source/ecs/manager.d
+++ b/source/ecs/manager.d
@@ -289,7 +289,7 @@ class EntityManager
                 dataSystem.handleEvent(input, *cast(Type*)data.event);
             }
 
-            static void setEventCallers(Sys)(System system){
+            static void setEventCallers(Sys)(ref System system){
                 enum allEventHandlers=__traits(getOverloads, Sys, "handleEvent").length;
                 system.m_event_callers = Mallocator.instance.makeArray!(System.EventCaller)(allEventHandlers);